Separate Specification from PenaltyModel Class

[m3ller]

Current Problem Currently, the Specification class is a super class of PenaltyModel. This is a bit confusing as Specification objects get passed to penaltymodel-mip, -maxgap, and -lp in order to generate a PenaltyModel object.

This also leads to issues between classical_gap and min_classical_gap in PR #74. (Most notably that the subclass classical_gap arg gets passed to the super's min_classical_gap).

Proposed Solution Refactor the code so that PenaltyModel is a subclass of BQM